Semiconductor components, such as wafers, dice and packages can include external contacts in the form of solder balls. For some components, such as chip scale packages, the balls can be arranged in a dense array, such as a ball grid array (BGA), or a fine ball grid array (FBGA). The balls provide a high input/output capability for a component, and permit the component to be surface mounted to a mating component such as a printed circuit board (PCB).
One conventional method for attaching the balls to a component substrate uses a solder reflow process. With this method the substrate can include bonding sites, such as bond pads, or land pads, on which layers of flux are deposited. A platen can be used to hold the substrate, while the flux is deposited on the bonding sites in a required pattern. After depositing the flux layers, the solder balls can be placed on the flux layers, and a convection furnace used to reflow the flux layers. After cooling, a permanent solder bond is formed between the bonding sites and solder balls.
Because the solder balls have a natural attraction for the flux layers, the alignment step is facilitated. One problem with this method is that during the heating step, the flux can liquefy prior to the balls. As the flux liquefies, the balls are free to move and can roll off the bonding site. This can cause missing and misaligned balls, and also defective components. Defects can lower throughput in a production process, and necessitate expensive rework procedures.
In order to maintain the balls in alignment with the bonding sites, a ball retaining plate is sometimes employed during the aligning and heating steps. For example, the ball retaining plate can include separate cavities for retaining each solder ball. A vacuum can also be applied to the cavities to provide a positive force for holding the balls in the cavities. U.S. Pat. No. 5,118,027 to Braun et al. discloses a reflow process in which a ball retaining plate and vacuum are used to hold the solder balls.
In general this method, can be performed on balls that have a diameter of about 0.012-in (0.305 mm) or larger. A center to center pitch of the balls can be about 0.018-in (0.457) mm. However, as the balls become smaller, and the spacing between the balls become tighter, it becomes more difficult to align and attach the balls.
Another problem with prior art aligning and attaching methods is the difficulty of fabricating ball retaining plates with the required feature sizes. For example, for fine ball grid array (FGBA) components, the balls can have a diameter as small as 0.005-in (0.127 mm), and a center to center pitch of only about 0.008-in (0.228 mm). It is difficult to make ball retaining plates with the required features sizes using conventional machining processes.
Balls can also be attached to bonding sites by laser reflow, soldering, brazing, welding, or applying a conductive adhesive. In each case some method must be employed to align and maintain the alignment of the balls to the bonding sites. A solder ball bumper, for example, employs a tool to precisely position pre-formed balls on a bonding site, and a laser to initiate reflow. However, solder ball bumpers, and similar apparatus, are relative expensive to purchase and maintain, and can have a limited throughput for volume semiconductor manufacture.
The balls, rather than being pre-formed members, can also be formed directly on the bonding sites using a deposition process such as screen printing, evaporation through a mask, or chemical vapor deposition. These deposition methods each have limitations with respect to the size and spacing of the balls, and also require relatively expensive equipment.
In view of the foregoing, there is a need in the art for improved methods and apparatus for forming contacts on semiconductor components.